The Anthropocene marks a new geological epoch, characterized by the dominance of anthropogenic forcings that have radically reshaped ecosystem functioning. Human activities now constitute the primary drivers of environmental change, profoundly altering the structure, dynamics, and resilience of both marine and terrestrial ecosystems. The cumulative effects of direct and indirect anthropogenic influences are particularly pronounced in coastal ecosystems, where human impact manifests through habitat destruction, eutrophication, pollution, overfishing, the introduction of invasive species, climate change, extreme events, and the disruption of trophic networks. These various stressors act over time, either as short-term or long-term disturbances, and frequently interact in additive and synergistic ways, undermining species, populations, ecosystems, and their capacity to provide essential ecosystem services. The Mediterranean Sea stands as a prime example of this phenomenon, being impacted by the full range of anthropogenic stressors. The synergistic interactions among these stressors often amplify their effects, resulting in more pronounced pressures on marine ecosystems.
The Mediterranean Sea has long been known for its vibrant marine forests, crucial to the structure and functioning of coastal ecosystems. Among these, the brown seaweed Gongolaria barbata, nicknamed golden pine seaweed (previously known as Cystoseira barbata) once thrived in shallow, sheltered reef habitats. This species plays a critical ecological role, forming complex three-dimensional habitats that support diverse associated faunal and algal communities. However, these habitats are under significant threat from both anthropogenic and herbivory pressures.
In this study, the long-term changes in the distribution of Gongolaria barbata in response to various disturbances have been assessed along the French Mediterranean coast, including Corsica and brackish lagoons, using historical data dating back to the 19th century. The current distribution was mapped through an extensive snorkelling survey that covered approximately 3,000 km of coastline. This study provides an invaluable baseline for future monitoring efforts. For the first time, an exhaustive map of the distribution of Gongolaria barbata populations has been produced at a very local scale (2:2 500) for the entire French Mediterranean coast. This mapping effort and the GIS analysis allows for an updated evaluation of the species' status according to the IUCN Red List criteria. Based on the findings, Gongolaria barbata should be classified as 'Regionally Extinct' (RE) in French Catalonia and Western Provence. In Languedoc, the species warrants a 'Vulnerable' (VU) classification under the IUCN criterion VU A2ac, as it is now extinct in the open sea but persists in certain coastal lagoons, where it faces substantial competitive pressure from introduced seaweeds, particularly Sargassum muticum. In the French Riviera, Gongolaria barbata should be classified as 'Critically Endangered' (CR) under the IUCN criterion CR A2ac, due to the loss of more than 75% of its historical localities, and because, although still present, the species no longer fulfils its former ecological role. Finally, in Eastern Provence and Corsica, Gongolaria barbata should be classified as 'Least Concern' (LC), reflecting the stability of its populations in these regions.
In regions such as French Catalonia and Western Provence, the primary factors contributing to the dramatic decline of Gongolaria barbata are overgrazing, habitat destruction, and uprooting. In contrast, habitat destruction emerges as the dominant driver in Languedoc, Eastern Provence, and the French Riviera. In Corsica, both overgrazing and habitat destruction appear to be contributing factors in roughly equal measure. Certain species of Cystoseira sensu lato, including G. barbata, have historically played a significant functional role in Mediterranean sublittoral reef ecosystems. As such, in the French Riviera, G. barbata is already considered functionally extinct, as the surviving populations no longer contribute meaningfully to ecosystem functioning; a situation also observed on Menorca Island (Spain).
The restoration of key species like Gongolaria barbata is being actively promoted internationally, aiming to reverse the observed declines in marine forests and ultimately re-establish the ecosystem services they provide. This initiative is supported by various global efforts, including the European Commission's Biodiversity Strategy to 2020 and 2030, as well as the United Nations Decade on Ecosystem Restoration (2021–2030), which seek to halt biodiversity loss and restore ecosystems worldwide.
Gongolaria barbata, the climax species of one of the habitats within the infralittoral rock ecosystem, characterized by photophilic algae, represents one of two stable states with barren grounds. All intermediate stages of succession, including colonization by turf algae, are possible. This ecosystem operates under top-down control, meaning it is regulated by predators, including top predators and sea urchin predators. Overexploitation of these predators leads to an unchecked increase in herbivores, such as sea urchins and the fish, Sarpa salpa, which in turn decimate the Fucales forests. The ongoing decline of these populations signals a profound and irreversible shift in the ecosystem, transitioning it into a barren-ground state. Restoring extinct or ecologically compromised populations is only meaningful if it is part of a comprehensive and integrated approach to managing human activities. Such restoration efforts must take place over large areas where the pressures have been sufficiently reduced to a level that the Fucales can once again thrive. One viable strategy is the active ecological restoration of these habitats. This approach can mitigate the adverse impacts of anthropogenic activities, thereby improving the chances of successful ecosystem rehabilitation. However, this requires a careful evaluation of grazing pressures in the targeted restoration zones, followed by regulatory actions tailored to reduce these pressures.
While bottom-up forces are crucial for understanding how ecosystems respond to disturbances, they are not sufficient by themselves. Therefore, top-down controls must also be considered to ensure optimal management and the effective restoration of marine forests. Protective measures, which represent a form of passive restoration, are often more cost-effective and easier to implement than active restoration efforts. However, there is an urgent need to develop and implement a regional strategy for regulating herbivore populations to protect marine forests and the essential ecosystem services they provide. In the event of significant decline, active ecological restoration, coupled with mitigation strategies, may offer a solution. It is important, however, to ensure that the species being restored were historically present in the area and that the causes of their decline have been identified and eliminated. Furthermore, restoration efforts should prioritize ecosystem functionality rather than focusing solely on the reintroduction of individual species, ensuring that the restoration is both species- and ecosystem-centered.